Implemented __builtin_expect for current cvs. This
was a Cygnus local feature done ages ago, but in a
moderately kludgy way. At the same time, I got to seriously
look at the block reordering code that Stan did earlier this
year. It's looking pretty good. Even without profile
feedback or explicit annotations,
estimate_probabilities seems to come up with
reasonable guesses a lot of the time. It'll be good when
Jason finishes fixing the debug info interactions so we can
turn this on for real.

Franz ran into yet another problem with the new flow code.
Seems the PPC backend is being naughty with eliminible
registers. Geoff was non-committal about fixing it, at
least right away. I suppose we can hack around the problem
for now.

Nasty structure passing bug uncovered in the sibcall code.
No fucking clue how to fix it properly. But if we did solve
it properly, we'd have automatically fixed the
REG_EQUAL backing store uncleanliness at the same
time. Should discuss alternatives with Law tomorrow.